1. Field of the Invention
The present invention relates to an image display device and a method for adjusting a color wheel index, and more particularly, to a device and method for adjusting a color wheel index value depending on a change of color temperature in a DLP optical system.
2. Description of the Related Art
A Digital Light Process (DLP) optical system compared to a Liquid Crystal Display (LCD) projection system employs a new display method using a light-reflecting element to achieve high definition of image quality.
The DLP optical system produces high definition and high brightness of image quality by selectively reflecting light and using an integrated circuit configured with thousands of light-reflecting elements.
The DLP optical system includes a Digital Micro-mirror Device (DMD) that is an integration of millions of micro mirrors depending on the resolution and signal-processing chips for controlling each mirror integrated in the DMD.
FIG. 1 is a view of a related art DLP optical system.
Referring to FIG. 1, the related art DLP optical system includes a lamp 10 for emitting light, a rod lens 11 through which the emitted light from the lamp 10 passes, a color wheel 12 for separating a white light passing through the rod lens 11 into red (R), green (G) and blue (B) lights, a condensing lens 13 for condensing the separated RGB lights provided by the color wheel 12, a prism 15 for reflecting the condensed lights toward a DMD 14, the DMD 14 for selectively reflecting the red, green and/or blue lights to generate appropriate color and projecting the reflected lights toward a projection lens 16, and the projection lens 16 for magnifying the reflected lights and directing the lights toward a screen 17.
Based on the above configuration, an operation of the related art DLP optical system will now be described. The white light emitted from the lamp 10 is concentrated by an internal curvature of a reflector and then the concentrated light passes through the light tunnel or the rod lens 11.
The rod lens 11 is provided by attaching four small and elongated mirrors facing one another. The light passing through the rod lens 11 is scattered and reflected, so that brightness distribution becomes uniform. The brightness of light that will be finally projected on the screen 17 needs to be uniform. The rod lens 11 performs this function and therefore it is an important optical element in a projection-type display device.
The light passing through the rod lens 11 is transmitted through the color wheel 12 for color separation. The color wheel 12 rotates depending on the vertical synchronization of an image.
Then the light passes through the condensing lens 13 and is reflected by the prism 15, so that the light is directed to the DMD 14. Depending on the angle of incidence, the prism 15 totally reflects or transmits the light.
The light incident on the DMD 14 is directed toward or out of the screen 17, depending on the on/off state of the micromirrors of the DMD 14 controlled in response to sampled pixel values.
The light reflected on the DMD 14 and directed toward the screen 17 passes through the projection lens 16. Then, the light is magnified by the projection lens 16 and is projected on the screen 17.
FIG. 2 is a view illustrating the color wheel 12 of the related art DLP optical system.
Referring to the FIG. 2, the color wheel 12 is driven by a DC motor 17 and RGB filters are attached to the color wheel 12.
To synchronize a rotational speed of the color wheel 12 and a color phase with a synchronous signal of an image signal, a color wheel index mark 18 is attached to the color wheel 12.
The R, G and B filters can be attached to the color wheel 12 in three equal parts of 360 degrees. In another example, two R filters, two G filters and two B filters can be attached in six equal parts of 360 degrees. In such a case, the DC motor 17 can reduce the rotational speed that is required for the light to pass through one color filter.
The size of the light passing through the rod lens 11 is called a “spot size”. When the light is on the boundary between the RGB filters, primary color light is not formed. Thus, such a light is not used in image representation. However, if the lights on all boundaries are collected, the white color light is formed, which is used to represent a white color.
The index mark 18 is used as a recognition index to synchronize the rotational speed of the color wheel 12 and the color phase with the synchronous signal of the image signal. The index mark 18 can be provided with a light-absorbing-black tape or paint.
The speed of the color wheel 12 rotated by the DC motor 17 and the color phase are synchronized with the synchronous signal by irradiating infrared rays toward the index mark 18 and detecting the reflected infrared rays.
Sometimes, however, the index mark 18 may be attached at a position of the color wheel 12 that is different from the preset position. In case that the index mark 18 is out of the preset position, the image signal is not synchronized with the light from the color wheel 12 so that high image quality is not produced. Therefore, specific software can be used to recognize the position of the index mark 18.
An adjustment value to recognize the position of the index mark 18 is called a color wheel index value. The DLP optical system controls a rotational speed and a color phase properly by using the color wheel index value.
FIGS. 4 to 6 illustrate chromaticity (color) coordinates for different temperature modes according to the related art. Particularly, FIGS. 4 to 6 show chromaticity coordinates of RGB colors for a cool mode, a medium mode, and a warm mode, respectively. In FIG. 4, a region 50 provides standard RGB color coordinates for obtaining the cool mode of color. In FIG. 5, a region 52 provides standard RGB color coordinates for obtaining the medium mode of color. In FIG. 6, a region 54 provides standard RGB color coordinates for obtaining the warm mode of color. As shown, the chromaticity (color) coordinates in the region 54 for the warm mode are generally higher than those for the cool mode in the region 50.
According to the related art, the color wheel index value is pre-set in one mode of color temperature (the medium mode, the cool mode, or the warm mode) and stays fixed at the set color wheel index throughout use. That is, generally at the manufacturing stage, the color wheel index is set to the medium mode. Thereafter the color wheel index cannot be changed and stays fixed at the set index value. An end user of the DLP optical system cannot change the color wheel index preset by the manufacturer or the like. As a result, if the color wheel index is set to be in the medium mode and then the color temperature of an image is changed into a cool mode or a warm mode, there occurs a problem of color bands since the color wheel index cannot be changed to correspond to the changed color temperature.
FIG. 3A is a view of an adjustment picture when the color wheel index is set in the medium mode, and FIG. 3B is a view of an adjustment picture showing undesired color bands 50 generated when the color wheel index is set in the medium mode but the color temperature is then changed into the cool mode. As can be seen from FIG. 3B, the undesired color bands 50 occur in some portions of the adjustment picture and degrade the picture quality.
As mentioned above, once the color wheel index is set in one mode of the color temperature and the color temperature is changed into another mode, there is a problem of color bands that deteriorate the picture quality.
According to the related art, as discussed above color bands occur as the color temperature mode is changed and this phenomenon is caused by difference between RGB gain and offset. Accordingly, the difference between the RGB gain and the offset needs to be adjusted properly. That is, once the color wheel index is set in the medium mode, the color bands shown in FIG. 3B occur due to the difference of the chromaticity coordinates for the different color temperature modes, which degrade the picture quality.